Severe Weather

Question 1

List the types of severe weather soundings discussed in class? (Hint 4)

Answer 1

Loaded Gun, Inverted V, Wet Microburst, Elevated Convection.

Question 2

Discuss the characteristics of the Inverted V sounding (Type IV). IE What does the sounding look like and how do atmospheric conditions change through the sounding?

Answer 2

· Named Inverted-V since the dewpoint depression decreases significantly with height
· Sounding has dry air (low RH) in lower troposphere with nearly saturated air (high RH) in middle troposphere

This sounding type is characterized by a relatively dry, well-mixed lower layer, with RH increasing with height, giving the appearance of an “inverted V.”

Question 3

What types of weather are associated with the Inverted V (Type IV) sounding and why?

Answer 3

· Convection tends to be high based since Convective Condensation Level is at a high elevation
· Most common severe weather: Strong winds > 58 mph; this is due to negative buoyancy of evaporationally cooled air aloft that causes it to accelerate toward the surface
· Gust fronts from inverted-V storms can have a large temperature gradient from one side to the other due to evaporative cooling
· Hail and tornadoes are not common due to the dry boundary layer, high cloud base and unorganized wind shear

It is typically associated with high based thunderstorms with vigorous, evaporatively driven downdrafts and microbursts.

Question 4

Where is the Inverted V (Type IV) sounding most common?

Answer 4

· Most common in interior Western U.S. (especially interior Southwest U.S.)

It is commonly observed during the summer season over the High Plains and the mountain and plateau regions of the Western U.S.

Question 5

Discuss the characteristics of the Loaded Gun (Type I) sounding. IE What does the sounding look like and how do atmospheric conditions change through the sounding?

Answer 5

· Severe weather sounding (large CAPE, very unstable LI)
· Large hydrolapse in mid-levels (mT air in boundary layer and capped by cT air)
· There must be an inversion above mT air

This sounding is characterized by a moist, fairly well-mixed layer of at least 100-150 hPa depth, separated from a dry layer above by a capping inversion. Lapse rates above the cap are typically nearly dry adiabatic.

Question 6

Where is the Loaded Gun (Type I) sounding most common?

Answer 6

· Great Plains during the spring severe weather season.

Question 7

What types of weather are associated with the Loaded Gun (Type I) sounding?

Answer 7

· Most common severe weather: Large hail, tornadoes, convective wind gusts of 58mph or greater
· If speed /directional wind shear and strong low-level jet are present on sounding, severe weather chances are enhanced

Question 8

Discuss the characteristics of the Wet Microburst sounding. IE What does the sounding look like and how do atmospheric conditions change through the sounding?

Answer 8

Mid-level dry air

· Similar to goal post sounding but with more moisture (higher precipitable water in sounding)

Question 9

Where is the Wet Microburst sounding most common?

Answer 9

· Sounding most commonly found east of Rockies

Question 10

What types of weather are associated with the Wet Microburst sounding?

Answer 10

· The dry air aloft will entrain into the downdraft and cause evaporative cooling. This increases the negative buoyancy and can result in microbursts and macrobursts
· If supercells develop they are most likely to be high precipitation supercells
· Most common severe weather: winds > 58mph, small hail near the 3/4” threshold, tornadoes possible (depends on low level shear and CAPE)

Question 11

Discuss the characteristics of the Type II sounding? IE What does the sounding look like and how do atmospheric conditions change through the sounding?

Answer 11

It is characterized by a deep moist, conditionally unstable layer with relative humidities of >60% from the surface up to 7 km AGL.

Question 12

Where is the two II sounding most common?

Answer 12

This sounding type is common in the tropics, but observed at times over much of the U.S. east of the Rocky Mountains, especially in the summertime over the Gulf Coast and Southeast.

Question 13

What types of weather are associated with a type II sounding and why?

Answer 13

There is no capping stable layer so that widespread convection is typically observed.

Question 14

What is the type III sounding and where is most common?

Answer 14

This sounding type is similar to Type 2, except 10-15°C cooler. It is commonly observed near cold-core, upper-level troughs and cyclones.

Question 15

What are hodographs used for?

Answer 15

The primary purpose of a hodograph is to reveal vertical wind shear.

Because the storm moves through its environment, the wind it experiences is often very different from the ground-relative winds. Storm-relative winds can also be calculated on a hodograph

Question 16

What is plotted on a hodograph and how is it plotted?

Answer 16

The hodograph is based on wind vectors, rather than wind barbs. To create a hodograph, wind vectors are plotted on a polar coordinate chart. Then their endpoints are connected.

Vertical wind shear is a description of how the velocity of the horizontal wind changes with height. We determine the vertical wind shear by taking the vector difference between the horizontal wind at two levels.

The total magnitude of vertical wind shear over a particular depth is an important factor in anticipating possible storm structure and evolution. Estimating total vertical wind shear is done by combining the lengths of all the shear vectors over a particular depth (the net length of the hodograph)

You can determine the direction of the mean wind shear vector (but not the magnitude) by drawing a line from the point that plots the surface wind to the point plotting the 6-km (20-kft)wind

Calculating the mean wind shear vector is simply a matter of averaging the x and y components of each of the single layer wind shear vectors

Question 17

What are the possible hodograph shapes and how do those shapes relate to storm morphology?

Answer 17

Strong straight shear tends to produce a pair of splitting mirror-image supercells.

Wind shear profiles with at least this much clockwise-curvature, common in the Great Plains, are responsible for producing dominant right-moving supercells.

Occasionally, the environmental shear creates a counterclockwise curving hodograph, which favor dominant, left-moving supercells.

Question 18

Describe how lightning forms.

Answer 18

1. First, we need a charged thunderstorm. While there is still considerable debate as to how a thunderstorm becomes charged, a popular theory is that collisions between graupel and ice crystals results in a transfer of negative charges to the graupel and positive charges to the smaller ice crystal. This may have to do with the difference in temperature between the warmer graupel. The thunderstorm becomes charged because the updrafts carries the lighter positively charged ice crystals to the upper portions of the storm and the heavier negatively charged graupel particles to the lower portions of the storm - thus creating a dipole.
2. Second, the negatively charged lower portions of the storm induces a positive charge at the earth’s surface. This creates an electrical potential between the bottom of the storm and the surface. When the electric fields exceed that of the breakdown voltage, a stream of electrons flow from the cloud base to the ground in steps of 50 to 100 m. These descending finger like paths are known as the stepped leaders.

The step leaders are very faint and are essentially invisible to the human eye. The stepped leader is important because it creates an ionized channel that will allow for the flow of charge during the remainder of the lightning stroke

3. When the stepped leader gets near the ground (~100 m or so) positive charges moves from the ground up toward the stepped leader –these are called Connecting Leaders. The connecting leaders may come from almost any pointed object on the ground: trees, antennas, grass, flag poles, telephone poles, people, really tall towers, etc.
4. Once the connecting leader touches the stepped leader, negative charged particles flow from the cloud to the ground and positive charges flow from the ground to the cloud. These charges flow along the ionized air channel formed by the stepped leader.

Question 19

Describe lightning climatology.

Answer 19

In the US, lightning most frequently occurs in Florida and along the Gulf coast. Across the world, the cast majority of lightning occurs over land with a high concentration near the equator in north Africa, northern south America, central America and southeast Asia.

Question 20

Describe Lightning Safety.

Answer 20

If outdoors, avoid water, high ground and open spaces. If indoors, avoid water, stay away from doors and windows, do not use a telephone. If lighting is in the area go indoors for at least 30 minutes after the last observed lightning or thunder.

Question 21

Describe the characteristics and severity of MCC’s

Answer 21

not answered

Question 22

Describe the climatology of MCC’s.

Answer 22

Average of 35 MCCs per year in the US.

Monthly Totals- 86 % occur during May , June, July, August

–The Great Plains experiences the most MCCs during the warm season.

–The Great Plains receives upwards of 18 percent of it’s annual warm season precipitation from MCC’s.

–The southeast experiences the most MCCs during the Spring season (primarily April ). On average, the Southeast receives 0 to 4 %.

–However, MCC activity is highly variable from year to year and warm-season precipitation totals are highly dependent upon MCC precipitation across much of the southeastern agricultural centers (e.g., Mississippi River floodplain)

Question 23

What are the key features associated with Squall Lines and Bow Echoes? (Hint 10)

Answer 23

Cold Pool which forces lift along a linear axis (gust front).

Updraft and updraft towers which would be the CB’s which form along the leading edge.

Downdraft which exists behind the gust front.

Stratiform Precipitation Area which is a large area of continuous rain.

Mid level low pressure which develops between the upflow and low level cold pool.
Rear Inflow Jet which forms in response to the mid level low. It is most common with trailing startiform systems. The strength depends on the strength of the mid level low which is regulated by positive and negative bouyancies between updraft and downdrafts generated by temperature differences between the updraft and cold pool. Some are forced to the surface by convergence with the updraft. Non-descending RIJ’s tend to promote a more longlived MCS.

MCV is a region of weak cyclonic rotation generated by pressure falls from widespread latent heat release.

Wake low develops behind the cold pool and MCS do to latent heat release in the stratiform region overhead and the descending rear inflow.

Line end vortices and book end vortices from in the mature stage - this is where the MCV may develop.

Rear Inflow Notch

Question 24

How do squall lines and bow echos develop?

Answer 24

In strongly sheared environments, the evolution of a squall line begins with an initially narrow line of strong convective cells, with light precipitation often extending downshear of the convective cores. Some of the cells may be supercells. As the system matures, the narrow line of strong cells persists, with bow-shaped segments of cells also beginning to develop. Lighter precipitation begins to extend somewhat rearward (upshear), but to a lesser extent than in weaker shears. In the dissipating stages, the leading cells weaken and become more scattered and the region of lighter precipitation extends even farther rearward (upshear). Squall lines tend to propagate in the direction of the 0-3 Km mean vertical wind shear vector at the speed of the cold pool.

As a squall line matures, it typically develops rotation at each end. The development of these line-end vortices is most apparent and significant for relatively short lines (less than 200 km, 110 n mi, in length). As we’ll discuss later, line-end vortices in close proximity to each other in bow echoes are given the special name bookend vortices. This development is schematically presented here for a 150 kilometer (80 n mi) long squall line evolving in an environment characterized by weak-to-moderate low-level shear.

Line-end vortices usually develop during the early to mature stages or between two to four hours into the lifetime of the convective system, just behind the zone of most active convection. When line-end vortices first develop, the cyclonic and anticyclonic vortices are often of nearly equal strength, promoting a symmetric, bowed shape in the precipitation field. However, if the vortices last for more than two to three hours (i.e., beyond four to seven hours into the lifetime of the system), the northern, cyclonic vortex tends to become stronger and larger than the southern, anticyclonic vortex. As this occurs, the convective system becomes asymmetric, with most of the stratiform precipitation region found behind the northern end of the system, and the strongest leading-line convective cells found near the southern end. In weak-to-moderate shear environments, the northern line-end vortex is typically observed to move rearward with time.

Bows develop as the rear inflow jet strengthens and often indicate very strong winds at the surface.

The optimal condition for the generation of new convective cells is when there is a balance between the horizontal vorticity produced by the cold pool and the opposite horizontal vorticity associated with the ambient low-level vertical wind shear on the downshear flank of the system.

Question 25

Squall Line and Bow Echo Severity.

Answer 25

Not answered.

Question 26

Name some Derecho types

Answer 26

Serial

- Produced by multiple bow echoes which are embedded in a squall line
- several hundred miles in length
- associated with a strong, migrating low pressure system
- found in the warm sector of a mid-level cyclone
- tend to occur with a well-defined 500mb trough upstream of a mid-latitude cyclone
- corresponds largely to cool season events (Sept - Apr)
- often found in the left exit region of a jet sreak
- 24% of events were serial
    - These are strongly forced events.

Progressive

- Characterized by a single bow-shaped system which propagates north of and parallel to an east-west boundary
- an upstream ridge is usually present for these events
- corresponds largely to warm season events (May-Aug)
- often found in or near the right entrance region of a jet streak
- 76% were progressive however, this may have been too high
    - These are weakly forced events.

Hybrid
combination of serial and progressive characteristics
may exhibit a zonal pattern or combinations of trough/ridge patterns
have no particular seasonal distribution

Question 27

Explain Derecho Climatology

Answer 27

the pattern of derecho occurrence is a distinct one
the greatest incidence tends to follow the mean position of the jet stream which is the demarcation line between warm and cold air at the surface, ie a frontal boundary

Question 28

How do derechos form?

Answer 28

Derechos
Defined as a widespread and long-lived windstorm associated with a band of rapidly moving showers and thunderstorms
Squall lines - derechos evolve from squall lines (linear MCS)
Bow Echoes - the radar signature most commonly associated with the derecho is the bow echo, indicated by a bulge in the squall line, bulge indicated the presence of faster moving winds, produced by the descent of the RIJ to the surface, the apex of the bow represents the area of strongest winds, the region where the most significant wind damage will occur.
Tend to develop with westerly or northwesterly flow aloft
form where 850mb dew points are highest, this produces the elevated convection which can contribute to downbursts
as they move, they migrate toward areas with high CAPE (often south of the boundary)
they will continue as long as sufficient CAPE is available and the shear is strong
There is lower/mid-level dry area which fosters evaporative cooling and further downward motion
additionally, there may be strong mid-level winds in place which can augment the RIJ
Tend to develop in environments where the LI is less than -6
81% produced tornadoes, the majority (92%) were EF2 or less

–Environments
Strongly forced derechos (Serial) had relatively cool and dry soundings with high shear and low CAPE (cold season)
Weakly forced derechos (Progressive) had warmer and moister soundings with low shear and high CAPE (warm season)
hybrids had mixed charachteristics

Question 29

What does the radar signature Hook Echo indicate?

Answer 29

It is a signature produced by precipitation held aloft that wraps around the mid-level mesocyclone. Since the mesocyclone has counterclockwise winds, the reflectivity signature of a hook echo will have a cyclonically shaped hook.

Question 30

What does the radar signature Kidney Bean Indicate?

Answer 30

HP supercells have a kidney bean shaped appearance on Doppler radar. Many HP supercells occur in multicell supercell clusters or when a classic supercell draws increasing amounts of moisture into the circulation. These are often termed rain-wrapped supercells. Tornadoes are generally more deadly when they are rain-wrapped since their approach is unknown until they are right on top of you. Hail tends to be smaller in association with HP supercells than in association with the other supercell types because:

Question 31

What does the Three-Body Scatter Spike indicate?

Answer 31

The three-body scatter spike appears as a streak of higher reflective which extends from the location of a hail core. These false echos occur because some of the radiation emitted from the radar strikes the hail core, is directed to the ground, back to the core and then back to the radar.

Question 32

What does the MARC signature indicate?

Answer 32

MARC is a Doppler velocity signature that serves as a precursor to the initial onset of damaging downburst winds in a squall line. Preliminary squall line case studies have shown a good correlation between values of MARC velocity differentials of at least 25 to 30 m/s (50 kts or greater) to successive damaging surface wind gusts. Strong mid-altitude radial convergence or MARC is often observed along the forward flank or leading edge of an organized bowing convective system.

Question 33

What does a rotating Comma Head indicate?

Answer 33

This is the “book end vorticy” which develops as a result of horizontal shear at the end of a squall line as it matures. There could sill be strong winds and even tornados in this region.

Question 34

What are the ingredients necessary for hail. (Hint 4)

Answer 34

Recipe:

1. cumulonimbus cloud
2. some type of hailstone embryo (ice crystal/snow or raindrop)
3. high liquid water content in the hail growth zone (-10 to -30 C).
4. strong and sustained updraft to keep the embryo in the hail growth zone for a sufficient length of time.High CAPE in the -10 to -30 Celsius region is good because it implies a strong updraft in the hail growth zone capable of suspending large hail.

Question 35

How does the hail grow?

Answer 35

Hailstone Growth

• -The embryo falls at a different speed than the supercooled cloud droplets.
• -Some droplets collide with and stick to the embryo (collision-coalescence process)
• -Depending on the temperature, different types of Wet Growth, Spongy Growth, Dry Growth
• -The layered structure in hailstone reflect different types of growth.

Question 36

Name and describe the different types of hail growth processes

Answer 36

Wet Growth
–All heat resulting from the collisions with and freezing of supercooled droplets warms the surface temperature of the hailstone to 0 C or greater and the hailstone become “wet.” The wet outer layer fills pores which causes a clear coating.

Spongy Growth
–Similar to wet growth except stones retain all of collected supercooled water (air bubbles retained and frozen)

Dry Growth
–All of supercooled water and ice crystals contacted by embryo are accreted. This process occurs very fast which traps air bubbles resulting in a translucent color.

Question 37

Describe hail climatology.

Answer 37

• -the climatological maximum of hail is not collocated in thunderstorm maximum
  
  • –The maximum number of thunderstorms is found in Florida
  • –The maximum number of hail storms is found in Wyoming, Colorado, and New mexico
  • –the melting level is too high in Florida to allow for significant hail to remain solid as it falls to the ground

Question 38

What is an MCS?

Answer 38

–Mesoscale convective systems (MCSs) refer to all organized convective systems larger than supercells. They normally result from some linear forcing mechnism.

Question 39

Name the types and sub-types of MCS’s. (4 classic types and 3 sub-types)

Answer 39

• -Some classic convective system types include:
  
  • squall lines, bow echoes and mesoscale convective complexes (MCCs)

Linear MCS subtypes

• -Trailing stratiform
• -Leading stratiform
• -Parallel stratiform

Question 40

Explain the 3 sub-types of MCS’s and what causes these variations.

Answer 40

–MCS structure affected by the speed and direction of the mid- and upper-level storm relative winds

MCS SubTypes
-Leading Stratiform (LS)
–typically slower-moving than other types, and is characterized by stronger mid- and upper- level storm relative flow (often described as rear-to front-flow) than any of the other types.
They produce the weakest cold pools.

• Parallel Stratiform (PS)
  
  • -a slightly less common form. The anvil and attendant stratiform precipation expand outward on either side of the intense convective line. They occur in situations with strong along-line storm relative flow, especially in mid-to upper -levels. They produce strong cold pools and therefore it is difficult to maintain this type for long time periods.
• Trailing Stratiform (TS)
• -This type has a sloped front-to rear flow produced by stronger system- relative flow in low-levels (and subsequent stronger low-level convergence along the leading edge). These MCSs exhibit the features where the cold pool provides most of the forcing for maintaining the system.

Question 41

Explain the development of the cold pool associated with MCS’s.

Answer 41

Boundary parallel shear tends to promote quicker cold pool merging.

Question 42

Explain the Lemon/Doswell model of tornado genesis.

Answer 42

First, the supercell begins with just a single rotating updraft. Next, low theata-e air near 7000 ft converges with the updraft which creates the rear flank downdraft. It is at this location, the interface between the updraft and downdraft, that the TVS forms. This new circulation begins to dominate the flow until it becomes the single rotation center. Eventually, this circulation moves under the rotating updraft.

Question 43

Explain the Klemp Model of tornado genesis.

Answer 43

– Klemp
model of a classic supercell storm looks nearly identical to the Lemon-Doswell but pattern of evolution followed by these models differs. lemon and Doswell have suggested that transition to tornadic phase is initiated by the RFD which forms at the mid-levels, creates a divided mesocyclone (zone of strong shear between the updraft and downdraft) which descends toward the ground.
klemp suggests a reverse sequence of events. Low-level rotation intensifies followed by the formation and lowering of the RFD which appears to be dynamically induced as strong low-level rotation lowers the pressure and draws the mid-level air downward.
Klemp - “the intensification (low level rotation) is stimulated by the baroclinic generation of strong horizontal vorticity along the low-level boundary of the cold pool forming beneath the forward flank of the storm. This horizontal vorticity is then tilted into the vertical and strongly stretched as the inflow enters the updraft. This horizontal vorticity is several times the magnitude of the environmental shear and is more favorably oriented to be tilted into the vertical”
If a storm encounters a pre-existing cold front or an outflow boundary from another storm, strong horizontal vorticity may be swept into the storm and stretched. This emphasizes how important it is for the radar operator to be aware of the location of boundaries.

Question 44

List the steps of the tornado life cycle (Hint 5 stages with 1 precursor).

Answer 44

0. Begins at stage 3 of the supercell life cycle.
1. Dust Whirl Stage
2. Organization Stage
3. Mature Stage
4. Shrinking Stage
5. Decay Stage

Question 45

Explain the Dust Whirl Stage.

Answer 45

–Dust whirl stage
circulation continuous from cloud to ground
vortex is visible as pressure is not yet low enough for condensation
visual identification - dust of light debris circulating at surface

Question 46

Explain the Organization Stage.

Answer 46

–Organization stage
appearance of condensation funnel
descends towards the ground as pressure lowers toward the ground
tornado may dissipate at this stage (w/rope like structure)

Question 47

Explain the Mature Stage.

Answer 47

–Mature stage
vortex is at its greatest size and intensity (can have a “wedge” shape)
wall cloud is very well organized (many cases - classic and LP, wall cloud rotation is visible)
development of feeder band from FFD into the wall cloud (tail cloud)
vortex breakdown is possible at this stage
development of a hydrodynamic low near base of tornado creates sinking air in the middle of the vortex. this sinking air can break the main vortex into 2-6 suction vortices or one primary helical structure
Hydrodynamic Low
Bernoulli’s Principle states - the pressure in a fluid decreases as the speed of the fluid increases

Question 48

Explain the Shrinking Stage.

Answer 48

–Shrinking stage
vortex diameter shrinks
rotation may increase (conservation of angular momentum)
vortex begins to tilt with the base lagging behind
wall cloud shrinks and disappears
tornado path hooks to the left with further RFD occlusion

Question 49

Explain the Decay Stage.

Answer 49

–Decay stage

vortex becomes stretched in a rope like form as the RFD occlusion is complete